Method of operating gas turbine engine with fan damage

ABSTRACT

Normal high load operation automatically varies nozzle area to maintain an optimum engine pressure ratio (EPR). An error signal representing fan damage is established by comparing the actual EPR to the predicted EPR. Compressor stalls are also monitored. In response to these signals a minimum nozzle area is set and modified. Automatic operation to hold EPR and afterburning is inhibited. Further signals representing satisfactory operation may reset the inhibiting action.

TECHNICAL FIELD

The invention relates to gas turbine engines with variable exhaustnozzles, and in particular, to accommodation of compressor fan damage.

BACKGROUND OF THE INVENTION

Gas turbine engines for aircraft often use variable area exhaustnozzles. Such engines may operate at low power in a base control mode,wherein the nozzle area is fixed. Throttle action by the pilot setseither a fuel flow rate or a engine RPM to be achieved, with pressuredistribution through the engine settling out at a new value. It isknown, however, that the thrust may be increased and the overallefficacy of engine operation improved by changing the area of the nozzleto an optimum condition for the new operating mode. If the nozzle closestoo much, it may cause a compressor stall, while if it opens more thanis necessary, over expansion within the discharge nozzle occurs.

It is accordingly known to measure the engine pressure ratio, which isthe ratio of pressure leaving the gas turbine to the pressure enteringthe compressor and to operate nozzle to maintain this parameter.Essentially, the pressure ratio is known for the engine design whichwill, for any particular RPM, provide reasonable tolerance from stallwith optimum thrust.

The fan, or first stage of the compressor, of an engine, is susceptibleto fan damage in various situations such as the ingestion of birds, iceor other foreign objects. The initial damage may result in a stallevent. In accordance with normal procedures the nozzle is opened to anincreased area until recovery from the stall, and then closed down toits normal operating position. Since fan damage has occurred, it isquite possible for the engine to continue to repeatedly stall, producingunstable operation. This is possible with a fixed nozzle condition, buteven more so when the engine is operating in the engine pressure ratiomode to achieve optimum thrust.

It is an object of the invention to detect and accommodate compressorfan damage, thereby effecting a proper choice of stall recovery action.

SUMMARY OF THE INVENTION

The gas turbine engine of the invention is a turbofan, with low and highpressure compressor, a turbine, an augmentor or afterburner and avariable area exhaust nozzle. A portion of the fan flow passes throughbypass ducts to the exit of the turbines. There is a known anticipatedengine pressure ratio for any operating air flow and nozzle areacondition which represents an undamaged compressor. It operates in theengine pressure ratio control mode with the nozzle being adjusted tomaintain a preselected engine pressure ratio at each high load operatingcondition. It is also capable of operating in a base control mode with afixed nozzle area.

In accordance with the objective to detect and accommodate fan damage,an enable logic disables the rest of the logic in certain situationswhere input data would be unreliable.

The relationship between engine airflow, exhaust nozzle area and enginepressure ratio is unique for a turbofan that is undamaged. Damage due toingestion of foreign objects results in reduced airflow rate of the fanand stall limit for a given rotor speed. Therefore, detection of fandamage is possible by comparing the actual engine pressure ratio for thedamaged fan to the predefined engine pressure ratio (EPR), airflow andexhaust nozzle area relationship for the undamaged fan. A percent EPRerror is thereby established based on that comparison.

In a fan damage and sensor error detection means, this error is comparedto tolerable errors. A relatively low sensor error detect is establishedwhere all potential sensor tolerances are on one side. A fan damagedetect is set at a higher level. Various responses occur depending onwhether the percent error signal is above the fan detect level, belowthe sensor error detect level, or in the band between the two. Each ofthese is combined with the signal indicating an immediately precedingstall and the action taken varies depending on whether or not there hasbeen an immediately preceding stall. Action other than stall recovery isnot taken until a quasi steady state operation is a achieved. Therefore,the phrase "in the presence of a stall" is the equivalent of "after animmediately preceding stall".

If the error signal obtained is above the fan damage detect level and astall has also occurred, the action taken depends on whether damage haspreviously been declared, this in turn being established by the settingof a fan damage flag. In the first instance, with no earlier fan damagedetected, a fan damage flag is set. A minimum area of the nozzle is setwith this area being selected as a function of the percent EPR error.EPR control is also stopped and after burning or augmentation isinhibited.

Should the EPR error be above the fan damage detect level in thepresence of a flag which represents early detected fan damage, thenozzle area is ratcheted to increase the previously selected minimumarea or the area measured at stall, whichever is larger, by anadditional percentage.

Should the percent error be above the fan damage detect level in theabsence of a stall, with no earlier damage detected, a sensor error flagis set and the nozzle area is set at a scheduled minimum area. EPRcontrol requests for exhaust nozzle areas less than the scheduledminimum are ignored, but augmentation is not inhibited.

Should the percent EPR level be above the fan damage detect level in theabsence of a stall, but with either the sensor error or the fan damageflag being set, no additional action is taken.

Should the percent EPR error be below the sensor error detect level,regardless of whether or not there was an immediately preceding stall,the sensor error and fan damage flags, if set, are reset. The system isreturned to EPR control and afterburning is permitted.

Should the percent EPR error be in the band between the sensor errordetect level and the fan damage detect level in the presence of a stall,operation is continued unchanged unless a sensor error or fan damageflag was set previously.

If the percent EPR error is in the band between the sensor error and thefan detect level in the presence of a stall and a flag has been setindicating either sensor error or fan damage error in the past, thenozzle area is increased a preselected amount with afterburneravailability being left where ever it was based on the earlieroperation.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a gas turbine engine with the prior art control schemeillustrated thereon;

FIG. 2 is a logic diagram of the fan damage detection scheme;

FIG. 3 is additional detail of the enable logic;

FIG. 4 is additional detail of the EPR error calculation;

FIG. 5 is additional detail of the fan damage detection logic; and

FIG. 6 is additional detail of a portion of the fan damage accommodationlogic.

FIG. 7 is a modification of FIG. 1 showing application of the fan damageaccommodation logic.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, a gas turbine engine, shown generally as 10, has afan or low pressure compressor 12 and high pressure compressor 14.Burners 16 are located upstream of turbine 18 with augmentorflameholders 20 followed by augmentor 22. Variable area exhaust nozzle24 discharges gas through nozzle area 26.

The known control system of FIG. 1 includes static pressure sensor 28,sensing static pressure at the low pressure compressor inlet. In thedesignation PS2 represents static pressure and the numeral 2 refers tothe location within the engine. A signal representing this pressurepasses through control line 30 to total pressure (PT2) calculator 32. Itis here combined with a corrected fan speed signal 34 producing acorrected total pressure signal passing through line 36 to a divisionpoint 38.

Pressure sensor 40 senses the pressure in the afterburner after theturbine exhaust, passing a total pressure signal through control line 42to division point 38.

At the division point, the signals are divided thereby obtaining apressure ratio signal by dividing the pressure PT6 by the pressure PT2.The signal is passed through control line 44 to comparison point 46where it is compared to an EPR set point signal 48. This set pointsignal is a preselected characteristic which is a function of thecorrected engine speed and the total inlet pressure PT2, corrected. Anydifference here results in a control error signal through line 50 whichwith appropriate proportional and integral action 52 passes tomultiplier 54 where AJ scheduling as a function of the base schedule isperformed. This signal then acts through actuator 56 to adjust nozzlearea 26 to achieve the set point EPR request.

The above described control loop which modifies the nozzle area toobtain a desired EPR is operative at high loads, for instance, greaterthan 90% power. At lower power, a base mode of operation is used whereina control signal 58 representing a desired nozzle area passes throughmultiplier 54 to actuator 56 to set the nozzle at the desired area. Inthis case, proportional and integral trim request for AJ less than thebase schedule are ignored such that the signal from 52 to multiplier 54will be 1 or greater. The base area schedule is a function of inlettotal temperature and inlet total pressure under normal operatingconditions with an additional increase for augmentation operation.

The above described control schemes are based on known engineaerodynamics and are established to maintain a reasonable tolerance froma compressor stall condition. When a stall does occur, the nozzle isopened for stall recovery and then returned to the preexisting controlposition. In the event of damage to fan 12, the aerodynamics of theengine change, increasing the probability of a stall. With such anoperating scheme, once recovery from a stall is accomplished, the enginereverts to its initial mode and if fan damage caused the stall, theengine would continue to stall and recycle resulting in unstableoperation.

Normal stall recovery procedures involving the opening of nozzle 24 areunimpeded by my invention. The fan damage detection processing unit 59(FIG. 7) incorporating the scheme shown generally in FIG. 2 operates todetect fan damage or sensor error which may cause stalling and to takeappropriate action. After normal stall recovery techniques are used, theengine operates with the large area nozzle until the fan damagedetection scheme described below performs its function. Operation thengoes to the mode as determined by the fan damage detection scheme.

Enable logic means 60 (shown in more detail in FIG. 3) disables thedetection scheme under conditions which would produce erroneous results.Typical inputs to enable the logic are shown in FIG. 3, wherein input 62represents that the appropriate pressure and speed sensors have notfailed. This is differentiated from the sensor error which produceserroneous readings which are accommodated later in the scheme. Signal 63requires a quasi-steady state operation to enable the system. Signal 64requires that operation be nonaugmented. Signal 65 requires that theoperation be within predetermined limits. For instance, the engine mustbe above a selected speed with the nozzle area below a selected size.The inlet pressure must be above a preselected value, such as, 0.4atmospheres to assure that the sensor is operating in a range where itstolerance would not adversely affect the system.

If all of the enablement conditions are met, an enabling signal passesthrough line 66 to EPR calculation means 70.

Input to the EPR error calculation means 70 includes the measured EPR71, the airflow 72 and the nozzle area 73. As indicated in more detailin FIG. 4, an undamaged engine has a known relationship 74 for anyparticular nozzle area with the anticipated engine pressure ratio beingknown as a function of airflow. Accordingly, from the input airflow andnozzle area an anticipated EPR is determinable. This is compared to themeasured EPR to obtain a percent error signal in accordance with theformula EPR anticipated minus EPR measured divided by EPR measured times100. A percent error signal accordingly is sent through control lines76. The signal also passes through control line 77 for purposes whichwill be described hereinafter, but for current purposes it passes to fandamage and sensor error detect means 80.

The fan damage and sensor error detection means 80 also has as input atotal pressure signal 82 representing the total pressure at thecompressor inlet. As shown in more detail in FIG. 5, the logic defines asensor error detect relationship 84 where the percent EPR error is shownas a function of the inlet pressure. This substantially represents theerror which would occur if the tolerance of all sensing apparatus wasoff the ideal in a single direction. A fan damage detect relationship 86is also established including some tolerance above the sensor errordetect curve, for instance, 5% greater. A dead band 87 occurs betweenthese two curves.

Within this detection means a percent error signal is compared to thedetect curves producing one of three signals depending on whether theerror is greater than the fan detect level 88, less than the sensorerror level 90, or between the sensor error and the fan detect levels92. As these control signals pass to the fan damage accommodation means,different actions are taken, not only with the three different signals,but in combination with each one of them as a function of whether or notthere has been an immediately preceding stall, and also whether or notfan damage or sensor error has previously been declared.

Looking first at a situation where the error is greater than the fandamage detect level, an immediately preceding stall exists, and damagehas not already been declared, the signal through line 88 passes to ANDbox 102 (FIG. 2). Stall detector 104 has passed a signal indicating astall through line 106 to memory 108 which retains informationindicating an immediately preceding stall. The YES signal for thepreceding stall passes through line 110 to AND box 102. The signalpasses to query box 112 questioning whether previous fan damage orsensor error has been declared. This would be noted by the establishmentof flags, but at this point we are assuming that no damage has earlierbeen declared.

Accordingly, a signal passes through control line 113 to set FD flag box114. This sets the flag for fan damage so that the logic later knowsthat fan damage was early declared. The signal then passes on to controlline 115 to fan damage accommodation area set logic 116, shown in moredetail in FIG. 6.

The early described percent EPR error signal passing through line 77from EPR error calculation means 70 is used at this point and hereinenters into the logic. Within the logic are three relationshipsrepresenting the area with respect to the percent EPR error signal.

Curve 118 represents the nozzle area to be set based on the percent EPRerror which is expected to avoid subsequent stalls. Curve 120 representsthe area in relationship to the percent EPR error which will produce 75%thrust. Curve 122 represents the area for EPR error calculation after astall and also the open limit for base mode operation.

The fan damage accommodation area set selects, based on the percent EPRerror established, a minimum area to be established for the nozzle. Thisis preferably the no stall line 118 for the lower errors and the 75%thrust lines at the higher errors where it produces a lower nozzle area.This provides an increased nozzle area attempting to prevent furtherstalls while producing 75% thrust or greater, but as will be seenhereinafter, if this area is not sufficient, further corrective actionwill be taken.

Since an EPR error of this magnitude would invalidate the EPR controlapparatus, EPR control is stopped and base mode control is establishedbased on the selected nozzle area. Afterburning is also inhibited. Insummary, in response to the high fan damage signal and an immediatelypreceding stall, a fan damage flag is set in the first instance, EPRcontrol is stopped and afterburning is inhibited.

Returning now to the detect means 80 with a greater than fan damagedetect signal 88, functions will be considered in response to apreceding fan damage determination. The presence of an immediatelypreceding stall is assumed so that the signals pass through AND box 102to the previous declaration box 112. In this case, the flag has been setpreviously and accordingly control signal passes through control line119 to increase area logic box 121. In accordance with the logic of thatbox, the nozzle area is increased a preselected amount, for instance 5%.No other change is made. If desired, a limit could be placed on themaximum area to be set.

In response to the above described logic the engine is operating on abase mode control scheme with a minimum nozzle area being establishedand for all practical purposes, maintained. The nozzle area may beincreased during transient conditions, for instance, an impending oractual stall recovery condition.

Returning again to detection logic 80, it will be assumed that an errorgreater than a fan detect level exists in the absence of a stall, andfurther in the absence of a previously set fan damage or sensor errorflag. The error signal 88 is combined with a no stall signal 123 in theAND box 124. This condition should be maintained for some time period,approximately 20 seconds, to further validate the detection accuracy.Since we are assuming that damage has not early been declared, thesignal passes through declaration box 126 and line 127 to set sensorerror flag 128. The sensor error flag is a record of the prior existenceof the present described operation. The signal continues through line129 to a nozzle area box 130.

Since the apparatus has indicated a high level of error, but no stallhas occurred, it is assumed that a sensor error exists. Accordingly,control logic 130 sets the minimum nozzle area to the base value of 0.28meter squared and stops EPR control. Afterburning is not inhibited.

Returning once more to detection logic 80, the greater than fan detectlevel signal, in conjunction with no stall, will be considered in lightof a previously set flag. The signals again pass through the AND box 124to the previous declaration box 126. If fan damage or sensor area haspreviously been declared, no action is taken.

The signal through line 90 of detect logic 80 represents a percent errorsignals which is lower than the sensor error detect level. If such a lowsignal is determined, nothing need be done where damage has never beendeclared. However, should damage have been previously declared, this lowerror level provides justification for resetting operation to avoid theinhibitions earlier placed on the system. Accordingly, the controlsignal from 90 passes through declaration box 132 to reset action box134 where any previously set fan damage or sensor error flag is reset.The signal further passes through control line 135 to logic box 136which releases the minimum nozzle area restriction, returns the systemto EPR control and permits afterburning.

As described herein, a control signal through line 92 is produced bydetect logic 80 when the error is in the band between the sensor errordetect level and fan damage detect level. It is intended that in thisarea, if there is no stall, that no action be taken. If a stall of firstoccurrence happens, it is desirable to continue the control as is sincethe fan damage detect level has not been exceeded. However, if theprevious damage was declared, as established by setting either the fandamage flag or the sensor error flag, it is desired to ratchet thenozzle area by increasing it 5%.

In order to achieve this, the control signal line 92 passes to AND box140 which requires the presence of a immediately preceding stall signal110 to send a control signal through control line 141. Previous damagequery box 142 operates such that in the event of no previous damagedeclaration, control signal through line 144 permits operation tocontinue as before.

If damage had previously been declared a signal through line 146 passesto increase nozzle area box 120 to increase the minimum nozzle area by5%.

FIG. 7 illustrates the incorporation of the logic into the gasic EPRcontrol system. The central processing unit 59 permits the controlsignal in line 50 to pass through until modification of the signal isimposed by the unit 59.

Burner pressure sensor 150 sends a signal to stall detector 152. In theevent of a stall a signal is sent through line 154 to the CPU 59.

The fan speed signal 34 is indicative of air flow and is sent as signal72 to the CPU 59. Total pressure signal passing through line 36 is alsosent to the CPU through line 82. A position signal 73 representingnozzle area is sent to the CPU. Also, the actual pressure ratio signalin line 44 is sent though line 71 to the CPU 59.

Steady state signal 63 and nonaugmentation signal 64 enter the CPU.Signal 62 entering the CPU indicates that the appropriate sensors haveno failure indication, while signal 64 indicates that operation iswithin preselected limits.

In response to logic 116, a signal for inhibiting augmentor orafter-burner operation is sent through control line 156 to block valve158.

We claim:
 1. A method of operating a gas turbine engine in a manner toaccommodate fan damage comprising:determining an actual EPR signal;establishing an EPR error signal by comparing said actual EPR signal toan anticipated EPR signal; establishing a fan damage signal, bycomparing said EPR error signal to a tolerable EPR fan damage error,when said EPR error signal exceeds said tolerable EPR fan damage error;comparing said EPR error signal to a pre-established nozzle area versusEPR error ratio relationship representing predicted stall limit with fandamage; determining the presence of an immediately preceding stall; andlimiting the minimum area of the engine nozzle to a correspondingpre-selected nozzle area in the presence of both said fan damage signaland an immediately preceding stall.
 2. A method as in claim 1 comprisingalso:comparing said EPR error signal to a tolerable EPR sensor error;and nullifying the step of limiting the minimum area when said EPR errorsignal is less than said tolerable EPR sensor error.
 3. A method as inclaim 1 comprising also:detecting a subsequent stall; ratcheting theminimum area of said nozzle to a higher value in response to saidsubsequent stall.
 4. A method as in claim 1 comprising also:inhibitingaugmentation of said engine in the presence of both said fan damagesignal and an immediately preceding stall.